Revision as of 17:26, February 22, 2014

Introduction

LLVM can be used as an alternative to GNU's compiler, GCC. The main benefit of using LLVM compilers instead of GCC is their lower memory usage, faster compile time and better diagnostics. There are some Benchmarks on the Clang and Phoronix homepages.

It may happen that some programs do not compile (like glibc) because they depend on GCC-specific language extensions [1] (this is why the whole BSD code can be compiled with LLVM but some GNU code cannot) or segfault after successful compilation with LLVM (like xorg-server) but after following this guide, the system will still be able to compile packages with gcc. So if something goes wrong, it can be switched back to gcc for the particular package by uncommenting lines in /etc/make.conf and the bug should be reported.

Further, using LLVM 3.0 and up, there is a third way to compile with LLVM: the dragonegg package creates a gcc-plugin, that uses LLVM's optimizers but parses the code and creates binaries with gcc, which means that everything that compiles and works with gcc should work with dragonegg also. This plugin can be enabled by using a single CFLAG. Since LLVM 3.0 the old llvm-gcc package is deprecated and replaced by dragonegg, so it will disappear from portage with llvm version 2.9.

LLVM Frontends

To be able to compile some sourcecode of a specific language, LLVM needs an appropriate frontend. There are clang, llvm-gcc and dragonegg in portage.

The goal of the Clang project is to create a new C, C++, Objective C and Objective C++ front-end for the LLVM compiler.

llvm-gcc is a modified version of gcc that compiles C/ObjC programs into native objects, LLVM bitcode or LLVM assembly language, depending upon the options. As written in the previous section, dragonegg replaced llvm-gcc in version 3.0.

So after installing llvm, clang and dragonegg, you will be able to choose between gcc and llvm whenever you like or use them both at the same time.

Install LLVM and its Frontends

Simply emerge the packages on ~arch systems. On arch systems you have to unmask some packages first. dragonegg requires gcc's lto USE-flag to be set and works with gcc 4.5 and gcc 4.6.

# emerge llvm clang dragonegg

Note, that for clang++ the C++ headers search path is hardcoded to the active gcc profile.
If you change the active gcc profile, or update gcc to a new version, you will have to remerge clang to update the search path.

To use dragonegg, run gcc as usual, with an extra command line argument -fplugin=/usr/lib/llvm/dragonegg.so
If you change the active gcc profile, or update gcc to a new version, you will have to remerge dragonegg to update the plugin.

After the installation, check which CPUs are supported by using the command

# llvm-as < /dev/null | llc -mcpu=help

and then add the following lines to /etc/make.conf (uncommenting the lines you need) to enable compilation via LLVM, adapting the march-option according to the previous command:

in /etc/portage/make.conf:

# LLVM
#CC="/usr/bin/clang"
#CXX="/usr/bin/clang++"
# llvm-gcc for C++ code and fortran
# llvm-gcc is deprecated and only used with LLVM 2.9
#CC="/usr/bin/llvm-gcc"
#CXX="/usr/bin/llvm-g++"
#CPP="/usr/bin/llvm-cpp"
#F77="/usr/bin/llvm-gfortran"
# Flags for clang: Insert your arch here instead of k8 and have a look at the manpage of clang for flag descriptions.
# Some gcc flags like -pipe and -pthread also work, though they might be ignored by clang.
#CFLAGS="-march=k8 -O2"
# Flags for dragonegg; just use all the gcc flags you like and append -fplugin=/path/to/dragonegg.so
#CFLAGS="-march=k8 -O2 -fplugin=/usr/lib64/llvm/dragonegg.so"

Note

Have a look at clang's manpages for additional information. If you get errors that your compiler cannot produce code, you should check your flags, e.g. don't use -O4 -flto -S or stuff like that; the examples above will work.

Using clang with portage

Although Gentoo package tree is not designed to be used with compiler other than GCC, clang can be enforced on most of the packages through CC and CXX variables.

Please note, however, that many of Gentoo packages still don't build with clang and a few don't work correctly after being built. That's why we suggest using /etc/portage/env file to enable the use of clang per-package.

In order to do that, first create a new environment override to use in /etc/portage/env/clang:

If you have app-portage/flaggie (package not on wiki - please add) installed, you can modify /etc/portage/package.env by running the following:

# flaggie app-foo/bar app-bar/baz +clang

Enabling link-time optimizations

The link-time optimization feature defers optimizing the resulting executables to linking phase. This can result in better optimization of packages but is unsupported in Gentoo, and many packages simply fail to build.

When using LTO, clang compiles units into LLVM byte-code rather than machine code. In order to support linking such object files, the gold linker must be installed and set as the default linker, as it does support plugins.

Similarly, ar needs plugin support as well. Sadly, binutils ar doesn't support passing '--plugin option before the actual command. Thus, we need to create a wrapper for it in /usr/local/bin/clang-ar:

Note that the link-time optimizations were indirectly enabled here via -O4. If you don't want to enable other optimizations enforced by -O3, please use -flto instead. You need to also pass optimization flags when linking because that's where clang needs them.

You may also need to adjust the libdir path to plugin. Newer (live) versions of clang add `-plugin` when linking automatically, so `-Wl,-plugin`… is no longer necessary.

Using clang with distcc

In order to use clang on distcc client, additional symlinks have to be created in /usr/lib*/distcc/bin: